Prior art valve metal, and particularly tantalum, flake powder for use in fabricating the anodes of electrolytic capacitors have been produced by mechanically milling powder in ball mills, vibratory ball mills, attritor mills and the like for the purpose of increasing the specific surface area of the particles. This approach has been broadly practiced and described in numerous patents such as U.S. Pat. Nos. 5,580,367; 4,940,490; 5,211,741; 5,261,942; 4,441,927; 4,555,268; 4,740,238 and 3,647,415 each of which is incorporated herein by reference.
Valve metal flakes made by prior art methods have been characterized as having a low to medium level of charge density, as typically reported in micro-farad volts per gram (CV/g), and the levels were considered to be the limit achievable by mechanical milling. It has been considered in the industry that mechanical deformation in steel milling machines can not be expected to achieve flakes with CV/g greater than about 20,000 micro-farad volts per gram of valve metal, preferably of tantalum, with a Brunauer-Emmett-Tell (BET) surface area of more than about 1 M2/g.
Milling is a very advanced art and those of skill in the art have considered the valve metal powders achieved by milling to have reached their pinnacle. While those of skill in the art have considered the mechanical limit to have been reached it has been surprisingly found, through diligent research, that high charge density powder can be achieved by producing high specific surface area with lower milling energy which is contrary to the expectations in the art. Under specific conditions, as set forth herein, a lower milling energy can actually achieve a high surface area with lower surface impurity thereby leading to a material which exceeds the properties previously thought achievable by mechanical milling.